Impacts of Disinfection Methods in a Granular Activated Carbon (GAC) Treatment System on Disinfected Drinking Water Toxicity and Antibiotic Resistance Induction Potential

Granular activated carbon (GAC) treatment followed by chlorination (GAC/Cl2) and chlorination followed by chloramination (Cl2/NH2Cl) are two methods utilized by drinking water treatment facilities to mitigate the formation of disinfection byproducts (DBPs) in treated water. However, the effectiveness of these methods in reducing the overall toxicity of drinking water, driven by DBPs, remains largely unknown. In this study, we evaluate the total toxicity of water samples from a pilot-scale GAC system with post-chlorination (GAC/Cl2), and occasionally pre-chlorination upstream of GAC (Cl2/GAC/Cl2), compared to water treated by chlorination followed by chloramination (Cl2/NH2Cl). The research was conducted at various bromide and iodide levels and across three GAC bed volumes. To assess DNA stress and oxidative stress in water extracts, we employed the yeast toxicogenomic assay and human cell RT-qPCR assay, along with the DBP analysis from in our previous study. Our results indicated that under environmental halogen conditions, GAC/Cl2 typically reduces both genotoxicity and oxidative stress in treated water more effectively than Cl2/NH2Cl and Cl2 treatment. However, Cl2/GAC/Cl2 does not consistently lower toxicity compared to GAC/Cl2. Notably, under high halogen conditions, Cl2/GAC/Cl2 fails to reduce genotoxicity and oxidative stress compared to samples without GAC treatment. Correlation analysis suggested that iodinated DBPs (I-DBPs) and nitrogenous DBPs (N-DBPs) were particularly associated with increased DNA stress and oxidative stress, indicating these classes of DBPs as significant contributors to the observed toxicity. While neither of these two categories of DBPs are regulated by the EPA, it appears that unregulated and unidentified DBPs significantly contribute to the genotoxicity and oxidative stress in drinking water. This research highlights the complex dynamics of water treatment processes and underscores the critical impact of unregulated DBPs on water toxicity.